Chapter 9 Classification of Physical Storage Media

Abstract

• Storage Hierarchy
• Magnetic Disks
• Disk Interface Standards
• Performance Measures of Disks
• Optimization of Disk-Block Access
• Flash Storage & SSD
• Storage Class Memory(NVM)

Classification of Physical Storage Media

• Can differentiate storage into:
  • volatile storage(易失存储): loses contents when power is switched off
  • non-volatile storage（非易失存储）:
    • Contents persist even when power is switched off.
    • Includes secondary and tertiary storage, as well as batter- backed up main-memory.
• Speed with which data can be accessed
• Cost per unit of data
• Reliability
  • data loss on power failure or system crash
  • physical failure of the storage device

Storage Hierarchy（存储级别）

• primary storage: Fastest media but volatile (cache, main memory).
• secondary storage: next level in hierarchy, non-volatile, moderately fast access time
  • also called on-line storage
  • E.g. flash memory, magnetic disks
• tertiary storage: lowest level in hierarchy, non-volatile, slow access time
  • also called off-line storage
  • E.g. magnetic tape, optical storage

Magnetic Disks(磁盘)

• Read-write head(读写头)
  • Positioned very close to the platter surface (almost touching it)
  • Reads or writes magnetically encoded information.
• Surface of platter divided into circular tracks（磁道）
  • Over 50K-100K tracks per platter on typical hard disks
• Each track is divided into sectors（扇区）.
  • A sector is the smallest unit of data that can be read or written.
  • Sector size typically 512 bytes
  • Typical sectors per track: 500 to 1000 (on inner tracks) to 1000 to 2000 (on outer tracks)
• To read/write a sector
  • disk arm swings to position head on right track
  • platter spins continually; data is read/written as sector passes under head
• Head-disk assemblies
  • multiple disk platters on a single spindle (1 to 5 usually)
  • one head per platter, mounted on a common arm.
• Cylinder（柱面） i consists of ith track of all the platters
• Disk controller(磁盘控制器) – interfaces between the computer system and the disk drive hardware.
  • accepts high-level commands to read or write a sector
  • initiates actions such as moving the disk arm to the right track and actually reading or writing the data
  • Computes and attaches checksums to each sector to verify that data is read back correctly
    • If data is corrupted, with very high probability stored checksum won’t match recomputed checksum
  • Ensures successful writing by reading back sector after writing it
  • Performs remapping of bad sectors

Performance Measures of Disks

• Access time(访问时间)– the time it takes from when a read or write request is issued to when data transfer begins. Consists of:
  • Seek time（寻道时间） – time it takes to reposition the arm over the correct track. (磁盘臂重定位)
    • Average seek time is ½ the worst case seek time.
    • 4 to 10 milliseconds on typical disks
  • Rotational latency （旋转延迟）– time it takes for the sector to be accessed to appear under the head.
    • Average latency is ½ of the worst case latency.
    • 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.)
• Data-transfer rate（数据传输率） – the rate at which data can be retrieved from or stored to the disk.
  • 25 to 100 MB per second max rate, lower for inner tracks
  • Multiple disks may share a controller, so rate that controller can handle is also important
    • E.g. SATA: 150 MB/sec, SATA-II 3Gb (300 MB/sec)
    • Ultra 320 SCSI: 320 MB/s, SAS (3 to 6 Gb/sec)
    • Fiber Channel (FC2Gb or 4Gb): 256 to 512 MB/s
• Disk block is a logical unit for storage allocation and retrieval
  • 4 to 16 kilobytes typically
    • Smaller blocks: more transfers from disk
    • Larger blocks: more space wasted due to partially filled blocks
• Sequential access pattern(顺序访问模式)
  • Successive requests are for successive disk blocks
  • Disk seek required only for first block
• Random access pattern（随机访问模式）
  • Successive requests are for blocks that can be anywhere on disk
  • Each access requires a seek
  • Transfer rates are low since a lot of time is wasted in seeks
• I/O operations per second (IOPS ，每秒I/O操作数)
  • Number of random block reads that a disk can support per second
  • 50 to 200 IOPS on current generation magnetic disks
• Mean time to failure (MTTF，平均故障时间) – the average time the disk is expected to run continuously without any failure.
  • Typically 3 to 5 years
  • Probability of failure of new disks is quite low, corresponding to a “theoretical MTTF” of 500,000 to 1,200,000 hours （57 to 136 years）for a new disk
  • An MTTF of 1,200,000 hours for a new disk means that given 1000 relatively new disks, on an average one will fail every 1200 hours(50 days)
  • MTTF decreases as disk ages

Optimization of Disk-Block Access

• Buffering: in-memory buffer to cache disk blocks
• Read-ahead(Prefetch): Read extra blocks from a track in anticipation that they will be requested soon
• Disk-arm-scheduling algorithms re-order block requests so that disk arm movement is minimized
  • elevator algorithm

• File organization
  • Allocate blocks of a file in as contiguous a manner as possible
  • Allocation in units of extents (盘区）
  • Files may get fragmented
    • E.g., if free blocks on disk are scattered, and newly created file has its blocks scattered over the disk
    • Sequential access to a fragmented file results in increased disk arm movement
    • Some systems have utilities to defragment the file system, in order to speed up file access
• Nonvolatile write buffers （非易失性写缓存） – speed up disk writes by writing blocks to a non-volatile RAM buffer immediately
  • Non-volatile RAM: battery backed up RAM or flash memory
    • Even if power fails, the data is safe and will be written to disk when power returns
  • Controller then writes to disk whenever the disk has no other requests or request has been pending for some time
  • Database operations that require data to be safely stored before continuing can continue without waiting for data to be written to disk
  • Writes can be reordered to minimize disk arm movement
• Log disk（日志磁盘） – a disk devoted to writing a sequential log of block updates
  • Used exactly like nonvolatile RAM
    • Write to log disk is very fast since no seeks are required
    • No need for special hardware (NV-RAM)

Flash Storage

• NAND flash - used widely for storage, cheaper than NOR flash
  • requires page-at-a-time read (page: 512 bytes to 4 KB)
    • Not much difference between sequential and random read
  • Page can only be written once
    • Must be erased to allow rewrite
• SSD(Solid State Disks)
  • Use standard block-oriented disk interfaces, but store data on multiple flash storage devices internally

• Erase happens in units of erase block
  • Takes 2 to 5 milliseconds
  • Erase block typically 256 KB to 1 MB (128 to 256 pages)
  • After 100,000 to 1,000,000 erases, erase block becomes unreliable and cannot be used
• Remapping of logical page addresses to physical page addresses avoids waiting for erase

• Flash translation table tracks mapping

  • also stored in a label field of flash page
  • remapping carried out by flash translation layer

• wear leveling(磨损均衡)- evenly distributed erase operators across physical blocks

Storage Class Memory(NVM)

• 3D-XPoint memory technology pioneered by Intel
• Available as Intel Optane
  • SSD interface shipped from 2017
    • Allows lower latency than flash SSDs
  • Non-volatile memory interface announced in 2018
    • Supports direct access to words, at speeds comparable to main-memory speeds

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